2. Instructor: Justin Hsia 6/20/2012Summer 2012 -- Lecture #31 CS 61C: Great Ideas in Computer Architecture C Arrays, Strings, More Pointers

3. Review of Last Lecture C Basics – Variables, Functions, Flow Control, Types, and Structs – Only 0 and NULL evaluate to FALSE Pointers hold addresses – Address vs. Value – Allow for efficient code, but prone to errors C functions “pass by value” – Passing pointers circumvents this 6/20/2012Summer 2012 -- Lecture #32

4. #include int main() { int *p; *p = 5; printf(“%d\n”,*p); } 3 Question: What is the result from executing the following code? Prints 5 ☐ Prints garbage ☐ Always crashes ☐ ☐

5. Great Idea #1: Levels of Representation/Interpretation 6/20/2012Summer 2012 -- Lecture #34 lw $t0, 0($2) lw $t1, 4($2) sw $t1, 0($2) sw $t0, 4($2) Higher-Level Language Program (e.g. C) Assembly Language Program (e.g. MIPS) Machine Language Program (MIPS) Hardware Architecture Description (e.g. block diagrams) Compiler Assembler Machine Interpretation temp = v[k]; v[k] = v[k+1]; v[k+1] = temp; 0000 1001 1100 0110 1010 1111 0101 1000 1010 1111 0101 1000 0000 1001 1100 0110 1100 0110 1010 1111 0101 1000 0000 1001 0101 1000 0000 1001 1100 0110 1010 1111 Logic Circuit Description (Circuit Schematic Diagrams) Architecture Implementation We are here_

6. Agenda Miscellaneous C Syntax Arrays Administrivia Strings More Pointers – Pointer Arithmetic – Pointer Misc 6/20/2012Summer 2012 -- Lecture #35

7. Assignment and Equality One of the most common errors for beginning C programmers a = b is assignment a == b is equality test When comparing with a constant, can avoid this by putting the variable on the right! – if (3 == a) {... } Correct – if (3 = a) {... } Won’t compile Comparisons use assigned value – if (a=b) is true if a≠0 after assignment ( b≠0 ) 6/20/2012Summer 2012 -- Lecture #36

8. Operator Precedence 6/20/2012Summer 2012 -- Lecture #37

9. Operator Precedence Use parentheses to manipulate Equality test ( == ) binds more tightly than logic ( &, |, &&, || ) – x&1==0 means x&(1==0) instead of (x&1)==0 *p++ means get value at address pointed to by p, then increment p *--p means decrement p to point to the previous data item and then use that value 6/20/2012Summer 2012 -- Lecture #38 For precedence/order of execution, see Table 2-1 on p. 53 of K&R

10. 9 Question: What is the output of the following code? char blocks[3] = {‘6’,‘1’,‘C’}; char *ptr = blocks, temp; temp = *++ptr; printf(“1: %c\n”, temp); temp = *ptr++; printf(“2: %c\n”, temp); 7878 ☐ 7171 ☐ 1 ☐ ☐ 1212

11. Agenda Miscellaneous C Syntax Arrays Administrivia Strings More Pointers – Pointer Arithmetic – Pointer Misc 6/20/2012Summer 2012 -- Lecture #310

12. Array Basics Declaration: int ar[2]; declares a 2-element integer array (just a block of memory) int ar[] = {795, 635}; declares and initializes a 2-element integer array Accessing elements: ar[num] returns the num th element – Zero-indexed 6/20/2012Summer 2012 -- Lecture #311

13. Arrays Basics Pitfall: An array in C does not know its own length, and its bounds are not checked! – We can accidentally access off the end of an array – We must pass the array and its size to any procedure that is going to manipulate it Mistakes with array bounds cause segmentation faults and bus errors – Be careful! These are VERY difficult to find (You’ll learn how to debug these in lab) 6/20/2012Summer 2012 -- Lecture #312

14. Accessing an Array Array size n: access entries 0 to n-1 Use separate variable for declaration & bound – Bad pattern int i, ar[10]; for(i=0; i<10; i++) {...} – Better pattern int ARRAY_SIZE = 10 int i, ar[ARRAY_SIZE]; for(i=0; i<ARRAY_SIZE; i++) {...} 6/20/2012Summer 2012 -- Lecture #313 Single source of truth! Bad Pattern Better Pattern

15. Arrays and Pointers Arrays are (almost) identical to pointers – char *string and char string[] are nearly identical declarations – Differ in subtle ways: initialization, sizeof(), etc. Key Concept: An array variable looks like a pointer to the first (0 th ) element – ar[0] same as *ar ; ar[2] same as *(ar+2) – We can use pointer arithmetic to conveniently access arrays An array variable is read-only (no assignment) (i.e. cannot use “ ar = [anything] ”) 6/20/2012Summer 2012 -- Lecture #314

16. Array and Pointer Example ar[i] is treated as *(ar+i) To zero an array, the following three ways are equivalent: 1)for(i=0; i<SIZE; i++) ar[i] = 0; 2)for(i=0; i<SIZE; i++) *(ar+i) = 0; 3)for(p=ar; p<ar+SIZE; p++) *p = 0; These use pointer arithmetic, which we will get to shortly 6/20/2012Summer 2012 -- Lecture #315

17. void foo() { int *p, a[4], x; p = &x; *p = 1; // or p[0] printf("*p:%u, p:%u, &p:%u\n",*p,p,&p); *a = 2; // or a[0] printf("*a:%u, a:%u, &a:%u\n",*a,a,&a); } Arrays Stored Differently Than Pointers 6/20/2012Summer 2012 -- Lecture #316 ?... ??4021 *p:1, p:40, &p:20 *a:2, a:24, &a:24 K&R: “An array__. name is not a variable”. a 24 ? 0 4 8 12 16 20 24 28 32 36 40 44 48… px

18. Arrays and Functions Declared arrays only allocated while the scope is valid: char *foo() { char string[32];...; return string; } An array is passed to a function as a pointer: int foo(int ar[], unsigned int size) {... ar[size-1]... } 6/20/2012Summer 2012 -- Lecture #317 BAD Must explicitly pass the size! Really int *ar

19. Arrays and Functions Array size gets lost when passed to a function What prints in the following code: int foo(int array[], unsigned int size) {... printf(“%d\n”, sizeof(array)); } int main(void) { int a[10], b[5];... foo(a, 10)... printf(“%d\n”, sizeof(a)); } 6/20/2012Summer 2012 -- Lecture #318 ??? sizeof(int *) 10*sizeof(int)

20. Agenda Miscellaneous C Syntax Arrays Administrivia Strings More Pointers – Pointer Arithmetic – Pointer Misc 6/20/2012Summer 2012 -- Lecture #319

21. Administrivia HW1 due Sunday night Lab 2 is up – don’t forget about extra credit Switching sections – how are things going? – Section 104 has room! – Sections 101 and 102 are crowded Website updates: – Discussion sheets & solutions in calendar – Staff HW0s (click on photos) 6/20/2012Summer 2012 -- Lecture #320

22. Agenda Miscellaneous C Syntax Arrays Administrivia Strings More Pointers – Pointer Arithmetic – Pointer Misc 6/20/2012Summer 2012 -- Lecture #321

23. C Strings String in C is just an array of characters char string[] = "abc"; How do you tell how long a string is? – Last character is followed by a 0 byte ( ‘\0’ ) (a.k.a. “null terminator”) 6/20/2012Summer 2012 -- Lecture #322 int strlen(char s[]) { int n = 0; while (s[n] != 0) n++; return n; } Array size here is 4 This means you need an extra space in your array!!!

24. C String Standard Functions Accessible with #include int strlen(char *string); – Returns the length of string (not including null term) int strcmp(char *str1, char *str2); – Return 0 if str1 and str2 are identical (how is this different from str1 == str2 ?) char *strcpy(char *dst, char *src); – Copy contents of string src to the memory at dst. Caller must ensure that dst has enough memory to hold the data to be copied – Note: dst = src only copies pointers 6/20/2012Summer 2012 -- Lecture #323

25. String Examples #include int main () { char s1[10], s2[10], s3[]=“hello”, *s4=“hola”; strcpy(s1,“hi”); strcpy(s2,“hi”); } Value of the following expressions? 6/20/2012Summer 2012 -- Lecture #324 sizeof(s1) 10 strlen(s1) 2 s1==s2 0 strcmp(s1,s2) 0 strcmp(s1,s3) 4 strcmp(s1,s4) -6 (s1 > s3) (s1 < s4) Point to different locations!

26. Throws an error ☐ Changes characters in string t to the next character in the string s ☐ Copies a string at address t to the string at address s ☐ ☐ 25 Question: What does this function do when called? void foo(char *s, char *t) { while (*s) s++; while (*s++ = *t++) ; }

27. Agenda Miscellaneous C Syntax Arrays Administrivia Strings More Pointers – Pointer Arithmetic – Pointer Misc 6/20/2012Summer 2012 -- Lecture #326

28. Pointer Arithmetic pointer ± number – e.g. pointer + 1 adds 1 something to the address Compare what happens: (assume a at address 100) Pointer arithmetic should be used cautiously 6/20/2012Summer 2012 -- Lecture #327 Adds 1*sizeof(char) Adds 1*sizeof(int) char *p; char a;int *p; int a; p = &a; printf(“%u %u\n”,p,p+1); 100 101100 104

29. Pointer Arithmetic A pointer is just a memory address, so we can add to/subtract from it to move through an array p+1 correctly increments p by sizeof(*p) – i.e. moves pointer to the next array element What about an array of large structs (objects)? – Struct declaration tells C the size to use, so handled like basic types 6/20/2012Summer 2012 -- Lecture #328

30. Pointer Arithmetic What is valid pointer arithmetic? – Add an integer to a pointer – Subtract 2 pointers (in the same array) – Compare pointers (, >=) – Compare pointer to NULL (indicates that the pointer points to nothing) Everything else is illegal since it makes no sense: – Adding two pointers – Multiplying pointers – Subtract pointer from integer 6/20/2012Summer 2012 -- Lecture #329

31. Pointer Arithmetic to Copy Memory We can use pointer arithmetic to “walk” through memory: 6/20/2012Summer 2012 -- Lecture #330 void copy(int *from, int *to, int n) { int i; for (i=0; i<n; i++) { *to++ = *from++; } We have to pass the size ( n ) to copy This works due to operator precedence

32. int main(void){ int A[] = {5,10}; int *p = A; printf(“%u %d %d %d\n”, p, *p, A[0], A[1]); p = p + 1; printf(“%u %d %d %d\n”, p, *p, A[0], A[1]); *p = *p + 1; printf(“%u %d %d %d\n”, p, *p, A[0], A[1]); } 31 Question: The first printf outputs 100 5 5 10. What will the next two printf output? 101 10 5 10 then 101 11 5 11 ☐ 104 10 5 10 then 104 11 5 11 ☐ 100 6 6 10 then 101 6 6 10 ☐ ☐ A[1] 510 A[0] p

33. Get to Know Your Staff Category: Cal 6/20/2012Summer 2012 -- Lecture #332

34. Agenda Miscellaneous C Syntax Arrays Administrivia Strings More Pointers – Pointer Arithmetic – Pointer Misc 6/20/2012Summer 2012 -- Lecture #333

35. Pointers and Allocation When you declare a pointer (e.g. int *ptr; ), it doesn’t actually point to anything yet – It points somewhere (garbage; don’t know where) – Dereferencing will usually cause an error Option 1: Point to something that already exists – int *ptr,var; var = 5; ptr = &var1; – var has space implicitly allocated for it (declaration) Option 2: Allocate room in memory for new thing to point to (next lecture) 6/20/2012Summer 2012 -- Lecture #334

36. Pointers and Arrays Want to access all of array of size n and test for exit by comparing to address one element past the array: int ar[10], *p, *q, sum = 0;... p = &ar[0]; q = &ar[10]; while (p != q) /* sum = sum + *p; p = p + 1; */ sum += *p++; Yes! C defines that one element past end of array must be a valid address, i.e. will not cause an bus error or address error 6/20/2012Summer 2012 -- Lecture #335 Is this legal?

37. Pointers and Structures Variable declarations: struct Point { int x; int y; struct Point *p; }; Point pt1; Point pt2; Point *ptaddr; Valid operations: /* dot notation */ int h = pt1.x; pt2.y = pt1.y; /* arrow notation */ int h = ptaddr->x; int h = (*ptaddr).x; /* This works too */ pt1 = pt2; 6/20/2012Summer 2012 -- Lecture #336 Cannot contain an instance of itself, but can point to one Copies contents

38. Pointers to Pointers Pointer to a pointer, declared as **h Example: 6/20/2012Summer 2012 -- Lecture #337 void IncrementPtr(int **h) { *h = *h + 1; } int A[3] = {50, 60, 70}; int *q = A; IncrementPtr(&q); printf(“*q = %d\n”, *q); *q = 60 506070 A q q

39. 38 Question: Struct and Pointer Practice Assuming everything is properly initialized, what do the following expressions evaluate to? struct node { char *name; struct node *next; }; struct node *ar[5]; struct node **p = ar;... /* fill ar with initialized structs */ address ☐ data ☐ invalid ☐ 1)&p 2)p->name 3)p[7]->next 4)*((*p) + 2) 5)*(p[0]->next) 6)(*p)->next->name

40. 39 Answers: Struct and Pointer Practice 1)&p address (ptr to ptr to ptr) “address of” operator returns an address 2)p->name invalid Attempt to access field of a pointer 3)p[7]->next invalid Increment p into unknown memory, then dereference 4)*((*p) + 2) data (struct node) Access array, move along it, then access struct 5)*(p[0]->next) data (struct node) This is tricky. p[0] = *(p + 0) is valid and accesses the array of pointers, where -> operator correctly accesses field of struct, and dereference leaves us at another struct. 6)(*p)->next->name address (char array) next field points to struct, access name field, which is, itself, a pointer (string)

41. Summary Pointers and array variables are very similar – Can use pointer or array syntax to index into arrays Strings are null-terminated arrays of characters Pointer arithmetic moves the pointer by the size of the thing it’s pointing to Pointers are the source of many bugs in C, so handle with care 6/20/2012Summer 2012 -- Lecture #340